An estimated one million people in the United States have pacemaker implants. Although pacemakers are being placed in younger populations, little is known about the longevity of the leads in the pacemakers. The long term reliability of lead components is, at best, uncertain. The uncertainty can be life threatening. Several lead models using polyurethane have exhibited degradation of the insulation, resulting from environmental stress cracking. A predisposition to such insulation failure may not be recognized prior to market release. Other leads have a small metal J shape retention located beneath the outer insulation. These metal components may fracture and erode through the insulation. The protruding metal fragment can puncture the heart and has been associated with the death of two patients and pericardial tamponade in at least five others. Approximately 21,000 of these lead models have been implanted in the United States, and approximately 42,000 world wide.
Detection of the fractures is problematic: pacing and sensing may be normal, and fractures are difficult to visualize even with fluoroscopy in a cardiac catherization lab. In one study of 156 patients, 35 (22.4%) had a definite fracture. In five patients who had a irregularity in the lead contour on the initial evaluation and who underwent repeat fluoroscopy approximately one month after the initial fluoroscopy, three were identified as having a definite fracture. Neither the age of the patients, the time since lead implant, nor the site of the fracture correlated with the incidence of retention wire failure. Although fractures involving portions of the retention wire extending beyond the insulation were easy to diagnose, fractures still enclosed in the insulation were found to be extremely subtle. The ability to visualize such fractures depends on the orientation of the lead in a given view. Even with the most sensitive method available, i.e., visual evaluation fluoroscopic images obtained at high frame rates, one in eight normal leads was shown to have a fracture on follow-up. Moreover, implantable devices can develop time dependent failure modes that simply cannot be modeled in the laboratory.
Pacemaker leads exhibit relatively smooth and continuous motion. Abnormalities in the lead, such as occult fractures and perhaps insulation degradation, will likely result in distortions in the motion, which could be revealed by 3D motion analysis. In addition, if the 3D motion analysis is coupled with stress analysis, it provides an even more sensitive evaluation of the status of the lead. Because patients are being followed with frequent imaging, the monitoring and quantitation of changes in 3D motion over time may thus facilitate detection of fractures and may be useful as an indicator of the possibility of future fractures.
There are other benefits to the development, in vitro testing, and in vivo evaluations of implants. New approaches are required to advance knowledge for accelerated testing for durability and reliability of physical components. This applies to design robustness of implants to include failure modes, patient variability, biological stability, computer aided design and manufacturing. New models are needed to reduce manufacturing empiricism (trial and error) as well as to enable in vitro performance. More biostability data are needed, especially relating to the performance of implant materials under dynamic situations and addressing the severity of the biologic environment in both short and long-term use scenarios.
There is dearth of information on long-term effects of implants in humans. Thus, the scientific community is faced with two related but separate problems. One, individuals with pacemakers need to be monitored and the status of their leads need to be evaluated accurately. Two, methods need to be developed by which to develop and evaluate new models in vitro and in vivo. The essential first step toward addressing both of these problems is the development of methods for determination of the 3D motion of pacemaker leads in vivo. A determination of the 3D motion of leads in populations of patients followed over time may provide useful information regarding progression of fractures. Further, the patient data could be combined (e.g., by parameterization) so as to lead to a greater understanding of the motions in the patient population and the development of in vitro modeling that truly mimics the in vivo lead motion. The techniques used to determine the motion of the pacemaker leads can also be used to evaluate the motions generated by the testing devices.
Tomographic techniques, such as CT, MRI, or ultrasound, yield only a single slice at a given point in time, and cannot provide instantaneous 3D positions of the entire lead. With the current technology, the only means to obtain accurate 3D motion throughout the heart cycle is to use biplane image acquisition techniques. To date, only Harrigan et al. have even mentioned a technique for pacemaker lead assessment in vivo (T. Harrigan, R. Kirkeeide, S. Jalal, T. Berveridge, B. D. Montgomery, H. B. Shih: Assessment of pacing lead curvature and strain with three dimensional reconstruction of biplane cineangiographic images in vivo. JACC 27:345A, 1996). The Harrigan disclosure is only an abstract and does not teach details about how to perform such a technique. Of more relevance to the present disclosure is the fact that Harrigan makes no mention of the problem of non-synchronous image generation.
The problems associated with the effects of non-synchronous acquisitions can be minimized to some extent by limiting image generation to the diastolic phase of the cardiac cycle where cardiac motion is at a minimum. However, such limited measurements during diastole of a little consequence to studies designed to evaluate the effects of stress on the pacemaker components, which stress is likely greatest during systole. There remains a need in the art, therefore, for an accurate and precise means of determining the location and movement of pacemaker components throughout the entire cardiac cycle.